The ultimate aim of converter refining is to blow molten steel having a low degree of superoxidation with high productivity and high yield. The decarburization behavior in converter refining is divided into a period I in which the decarburization rate is determined by a flow rate of oxygen supplied to molten iron in a region where the molten iron has a high carbon concentration, and a period II in which the decarburization rate is determined by a mass transfer rate of carbon in molten iron in a region where the molten iron has a low carbon concentration.
To improve the productivity in converter refining, it is required to increase the decarburization rate in the period I which occupies a large part of the refining time. For that purpose, the flow rate of oxygen supplied to molten iron requires to be increased in principle. However, the oxygen flow rate in a general top-and-bottom blowing converter has an upper limit of about 4 (Nm.sup.3 /ton/min). If the oxygen flow rate is increased beyond the upper limit value, violent splashes would come out, the amount of dust would be increased, and a phenomenon called slopping would occur. The occurrence of those phenomena reduces yield of molten steel, increases deposition of skull onto the top of the converter, and increases the amount of waste slag beneath the converter. Accordingly, problems of prolonging a non-blowing time such as taken to remove the skull and clean the ground beneath the converter and lowering the productivity are rather encountered.
There are known several techniques of pressurizing a converter for the purpose of increasing the oxygen flow rate and suppressing the occurrence of dust during the period I. As described below, however, any of the known techniques is not sufficient to provide satisfactory operating conditions.
For achieving improved yield of molten steel, it is needed not only to reduce the occurrence of dust and splashes during the period I, but also to suppress the iron oxidation loss occurred during the period II. In the region where molten steel has a low carbon concentration during the period II, an iron oxidation loss occurs because the molten steel is brought into a superoxidated state, and iron is oxidized and released into slag. Superoxidation of molten steel increases the amount of (T.multidot.Fe) in slag (i.e., total iron components contained in slag in the form of iron oxides or iron), and also increases an oxygen concentration in the molten steel. This gives rise to an additional problem that a large amount of deoxidizing agent is required and purity of the molten steel is considerably deteriorated due to deoxidation products generated in a large amount.
For suppressing superoxidation during the period II, it is conceivable in principle to reduce the oxygen flow rate and to increase stirring intensity. A reduction of the oxygen flow rate however prolongs the refining time, and therefore accompanies a problem that an improvement of the productivity cannot be achieved at the same time. Also, an increase of bottom-blow stirring intensity results in an increase of the stirring gas cost. An increase of the stirring gas cost can be suppressed by holding low the stirring intensity during the period I and increasing the stirring intensity only during the period II. However, due to lack of technology for remarkably changing the bottom-blown gas flow rate at the same tuyere, this method raises another problem that the wearing rate of bottom-blow tuyere bricks is increased.
Meanwhile, there are known several techniques of pressurizing the interior of a converter for the purpose of increasing the oxygen flow rate and suppressing the generation of dust. However, any of the known techniques is not sufficient to provide satisfactory operating conditions as follows.
Japanese Examined Patent Publication No. 43-9982 discloses an iron refining method comprising the steps of placing both an iron charge and a slag making component in a top blown converter, introducing oxygen through a lance positioned in the converter, causing the oxygen to flow over the surface of the iron charge located below the lance, thereby developing a refining reaction to remove carbon from iron and to generate a reactor gas, causing the reactor gas to flow from the converter to a gas collecting device, providing pressure adjusting means for controlling the gas velocity, and holding a close relation between the iron charge and the pressure adjusting means so that essentially all of the gas passes the pressure adjusting means. In addition, the pressure adjusting means is controlled so as to provide at least one atmospheric pressure within the converter when the iron charge is refined with the introduced oxygen.
The technique disclosed in the above publication is featured in that a carbon dioxide production ratio (post combustion rate) raises, and the amount of dust is reduced because a mass flow rate of the waste gas is lowered. The disclosed technique however contains no quantitative restrictions with regard to the oxygen flow rate and the relationship between impingement energy of a top-blown oxygen jet upon the bath surface and pressure, which greatly affect the post combustion rate and the amount of dust generated. Further, the disclosed technique relates to a top blown converter, and greatly differs in basic conditions from refining with a top-and-bottom blowing converter. Accordingly, a converter cannot be operated as a pressurized converter based on the disclosed invention alone.
Japanese Unexamined Patent Publication No. 2-205616 discloses a highly-efficient converter steelmaking method for refining iron materials, such as molten iron and scraps if necessary, to molten steel, wherein the interior of a converter is pressurized to 0.5 kgf/cm.sup.2 or more, the relationship between a total amount W (t/ch) of molten pig iron and scraps both charged into the converter and an inner volume V (m.sup.3) of the converter shell is set to satisfy W&gt;0.8 V or 0.8 V.gtoreq.W.gtoreq.0.5. V, and an oxygen flow rate U (Nm.sup.3 /min.multidot.t) into the converter is set to satisfy U.gtoreq.3.7. This publication explains that the occurrence of slopping and spitting was suppressed and high yield was obtained under pressurization.
However, the above-cited publication does not discuss the conditions for suppressing the occurrence of slopping and spitting in relation to the oxygen supply condition and the relationship between stirring intensity and the pressurizing condition. It is therefore impossible to carry out the operation of a pressurized converter based on the invention disclosed in the above-cited publication alone. In a top-and-bottom blowing converter, which is subjected to strong stirring intensity, particularly, slopping hardly occurs even at normal pressure under the conditions of the comparative example described in the above-cited publication. Thus, because of great difference in basic conditions, it is difficult to obtain the pressurized operating conditions for the top-and-bottom blowing converter from the invention disclosed in the above-cited publication.
Moreover, the above-cited publication does not explain a method for operating the converter under the condition of low carbon concentration during the period II, which is most important from the viewpoint of suppressing superoxidation and improving yield.
Japanese Unexamined Patent Publication No. 62-142712 discloses a steel- and iron-making method in a converter or a smelting reduction furnace, wherein the internal pressure of the converter or the smelting reduction furnace is set to a level higher than the atmospheric pressure, particularly in the range of 2-5 kg/cm.sup.2, so that the linear velocity of post combustion gas is lowered.
The invention disclosed in the above-cited publication intends to lower the velocity of rising flow of post combustion gas in slag under pressurization and to prolong a heat-exchange time between the gas and the slag, thereby improving the heat efficiency through the slag. The disclosed invention explains that the interior of the converter or furnace is pressurized to 2-5 kg/cm.sup.2, but contains no restrictions with regard to the amount of slag, the amount of post combustion gas generated, the oxygen flow rate, the height of a lance, the depth of a cavity, etc. which affect the heat-exchange time between the gas and the slag, in spite of that the heat-exchange time dominates the heat efficiency in accordance with the principles of the disclosed invention. It is therefore impossible to carry out the operation of a pressurized converter based on the disclosed invention alone. In particular, because an embodiment of the disclosed invention concerns with a top blown converter, basic conditions are greatly different between the disclosed invention and the case of employing a top-and-bottom blowing converter in which slag forming is hard to develop due to strong stirring intensity, or the case of blowing molten pig iron prepared through the hot metal pretreatment process in which the amount of slag is small. It is hence difficult to obtain the pressurized operating conditions for the top-and-bottom blowing converter from the disclosed invention.
Japanese Unexamined Patent Publication No. 2-298209 discloses a pressurized iron-containing-cold-material melting converter steelmaking method comprising the steps of supplying an iron-containing cold material, a carbon material and oxygen to a specific melting converter in which a source of molten iron is present, producing high-carbon molten iron in an amount equal to total of a predetermined amount of the source molten iron in the specific melting converter and a predetermined amount of molten iron to be refined in a separate specific refining converter, and obtaining molten steel having desired components by blowing the high-carbon molten iron, as materials, with oxygen in the specific refining converter, wherein the internal pressure of the specific melting converter is controlled in accordance with the following formula to thereby achieve a remarkable reduction of the amount of dust generated in the specific melting converter; EQU P.gtoreq.1.15+0.3{[% C]-25}25.gtoreq.[% C].gtoreq.5
symbol P: internal pressure (atm) of the specific melting converter, and PA1 [% C]: C content (weight %) of molten iron in the specific melting converter. PA1 P: converter internal pressure (kg/cm.sup.2) PA1 F: top-blown oxygen flow rate (Nm.sup.3 /ton/min) PA1 Q: bottom-blown gas flow rate (Nm.sup.3 /ton/min) PA1 Wm: amount of molten steel (ton) PA1 L: cavity depth (mm) of molten iron PA1 LG: distance (mm) between the lance tip and the static molten iron surface PA1 P.sub.O : absolute pressure (kgf/cm.sup.2) at the nozzle inlet PA1 P.sub.OP : nozzle absolute pressure (kgf/cm.sup.2) at correct expansion PA1 M.sub.OP : discharge Mach number (-) at correct expansion PA1 d: nozzle throat diameter PA1 S.sub.e : area (mm.sup.2) of the lance nozzle outlet PA1 S.sub.t : area (mm.sup.2) of the lance nozzle throat PA1 P: atmosphere absolute pressure (kgf/cm.sup.2) at the lance nozzle outlet PA1 P.sub.OP : lance nozzle absolute pressure (kgf/cm.sup.2) at correct expansion PA1 M.sub.OP : discharge Mach number (-) at correct expansion PA1 P: atmosphere absolute pressure (kgf/cm.sup.2) at the lance nozzle outlet PA1 P.sub.OP : lance nozzle absolute pressure (kgf/cm.sup.2) at correct expansion PA1 S.sub.t : area (mm.sup.2) of the lance nozzle throat PA1 P.sub.O : absolute pressure (kgf/cm.sup.2) at the lance nozzle inlet PA1 F.sub.O2 : oxygen gas flow rate (Nm.sup.3 /h) PA1 .epsilon.: flow rate coefficient (-) (usually in the range of 0.9-1.0)
The invention disclosed in the above-cited publication utilizes the facts that impingement energy of a top-blown oxygen jet upon the bath surface is reduced under pressurization, and the volume of generated CO gas is also reduced under pressurization. Because CO tends to generate in a larger amount as the molten iron has a higher carbon concentration, the pressure is set to a higher level depending on the carbon concentration. However, the above formula is applied to the C content ranging from 2.5 to 5%, and therefore cannot be applied to converter refining aiming at decarburization. Also, the generation rate of dust depends on not only merely pressure but also the oxygen flow rate to a large extent, and the oxygen flow rate is an important factor which dominates the productivity of a converter for melting an iron-containing cold material. Nevertheless, the disclosed invention contains no quantitative restrictions with regard to the oxygen flow rate and the relationship between impingement energy of a top-blown oxygen jet upon the bath surface and pressure. Additionally, the disclosed invention greatly differs in basic conditions from converter refining aiming at decarburization. It is therefore impossible to carry out the operation of a pressurized converter based on the disclosed invention alone.
Furthermore, any of the above-described known arts does not disclose a method for operating a converter in the low-carbon region during the period II, which is most important from the viewpoint of suppressing superoxidation and improving yield. In the period II, particularly, it is impossible to suppress superoxidation and to improve yield, while improving the productivity, unless such conditions as the top-blown oxygen flow rate and stirring intensity due to bottom blowing are properly controlled in addition to the internal pressure of the converter.
Conventionally, .epsilon. defined by the following formula (1) is used as stirring energy due to bottom blowing ("Tetsu to hagane", Vol. 67, 1981, p. 672 ff.), and there is known the relationship between a BOC value and a decarburization characteristic of a converter through a homogenous mixing interval .tau. determined by the following formula (2) ("Tetsu to haganew", Vol. 68, 1982, p. 1946 ff.). EQU .epsilon.=(371/Wm).multidot.Q.multidot.T.multidot.{log(1+(9. 8.multidot..rho..multidot.H/P).multidot.(10.sup.-4))} (1) EQU .tau.=540.multidot.(H/0.125).sup.2/3.multidot..rho..sup.1/ 3.multidot..epsilon. (2) EQU BOC={F/(1/.tau.)}x[%C] (3)
In these formulae, Q is the bottom-blown gas flow rate (Nm.sup.3 /ton/min), T is the temperature (K) of molten steel, .rho. is the density of molten steel, H is the bath depth, P is the internal pressure (kg/cm.sup.2) of a converter, F is the top-blown oxygen flow rate (F: Nm.sup.3 /ton/min), [% C] is the carbon concentration, and Wm is the amount of molten steel (ton).
From the above relationships, it was estimated that in the case of the converter having the bath depth of 1-2 m, for example, even when the converter internal pressure is raised from 1 kg/cm.sup.2 to 3 kg/cm.sup.2, effects upon .epsilon. and BOC are not remarkable and metallurgical characteristics are not greatly affected.
On the other hand, the following formula (4) is employed in calculating the depth of a cavity formed by top-blown gas (Kiyoshi Segawa: "Iron Metallurgical Reaction Engineering", 1977, Nikkan Kogyo Shimbun, Ltd.), but the effect of the converter internal pressure is not included in the formula (4). EQU L'=L.sub.h.multidot.exp(-0.78h/L.sub.h)L.sub.h =63.0(F'/nd).sup.2/3 (4)
In these formulae, L' is the cavity depth (mm) calculated by the above formula (4), h is the distance between the lance and the steel bath surface, F' is the top-blown oxygen flow rate (Nm.sup.3 /Hr), n is the number of nozzles, and d is the nozzle diameter (mm).
Also, for post combustion, there have been proposed relation with respect to L' resulted from the above formula (4) and relation with respect to (X-H.sub.c)/d, i.e., a ratio of the difference between the distance X from the lance tip to the bath surface and the length H.sub.c of a supersonic core to the nozzle diameter d ("Tetsu to hagane", Vol. 73, 1987, p. 1117 ff.). With regard to the latter relation, particularly, it is suggested that CO in the atmosphere is caught up into an oxygen jet and is subjected to post combustion for conversion into CO.sub.2 in a peripheral region of the jet where the velocity of the jet is relatively low. The article however does not describe changes depending on the converter internal pressure.
For the effect of the pressure upon the cavity depth, the behavior in a depressurized state is reported ("Tetsu to hagane", Vol. 63, 1977, p. 909 ff.). This article explains that the cavity depth is abruptly increased by decreasing the pressure. In other words, the article shows the result obtained at the atmospheric pressure or below, but contains no suggestions about the behavior in a pressurized state. If the result obtained at the atmospheric pressure or below is extrapolated to a region pressurized above the atmospheric pressure, the cavity depth is very small.